[SCSI] libfcoe: fix checking of conflicting fabrics in fcoe_ctlr_select()

The check for conflicting fabrics in fcoe_ctlr_select()
ignores any FCFs that aren't usable.  This is a minor
problem now but becomes more pronounced after later patches.

Signed-off-by: Joe Eykholt <jeykholt@cisco.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>